1. Field of the Invention
The present invention generally relates to filters, and more particularly, to a filter including acoustic wave filters connected in parallel.
2. Description of the Related Art
Recently, cellular phones and potable information terminal equipment have rapidly become widespread due to the progress of mobile communication systems. For example, the cellar phone terminals use a frequency range as high as 800 MHz to 1.0 GHz or 1.5 GHz to 2.0 GHz. In the communication fields as mentioned above, acoustic wave filters such as surface acoustic wave (SAW) filters are used. There is a demand for a SAW filter having a wide band. For example, some cellular phone terminals require a wide multi-band SAW filter capable of covering communication bands of two or more communication systems.
However, there is a difficultly in realizing filters capable of two or more bands and having low insertion loss. In view of the above circumstance, there is a proposed filter equipped with multiple acoustic wave filters that are connected in parallel and have bands of respective communication systems.
Japanese Patent Application Publication No. 11-68512 discloses a filter that uses two acoustic wave filters and has a single input and two unbalanced outputs. Japanese Patent No. 3480445 discloses a filter that uses two acoustic wave filters and has a single input and two balanced outputs. The two acoustic wave filters are connected in parallel with regard to a single input. It is required that one of the two filters has a high impedance in the pass band of the other filter in order to realize a filtering operation such that the two filters allow signals to pass in the respective pass bands and block signals outside of the respective pass bands.
In order to realize the above filtering operation, the above applications propose the use of a series resonator inserted between the acoustic wave filters and the input (common) terminal. The series resonator realizes an impedance adjustment such that one of the two filters has high impedance in the pass band of the other filter.
However, the series resonator increases the insertion loss and prevents downsizing of the filter.